The present invention relates to an air-fuel ratio closed-loop control method and apparatus for an internal combustion engine.
It is a well-known practice to provide an internal combustion engine with an air-fuel ratio closed-loop control system. Such a system calculates an air-fuel ratio correction factor responsive to a detection signal fed from a concentration sensor. The sensor detects the concentration of a particular component contained in the exhaust gas. An example of such a sensor is an oxygen concentration sensor (O.sub.2 sensor) for detecting the concentration of oxygen in the exhaust gas. The air-fuel ratio closed-loop control system corrects the amount of the fuel injected into the engine according to the calculated correction factor so as to control the air-fuel ratio to the desired value.
One type of air-fuel ratio closed-loop control system controls the air-fuel ratio if the O.sub.2 sensor is active even when the engine is warming up.
During warm-up, engines are generally supplied with an amount of fuel greater than usual (warm-up increment correction) and the air-fuel ratio is set at a richer condition than after warm-up. Therefore, when air-fuel ratio closed-loop control is carried out during warm-up, the air-fuel ratio correction factor is maintained at a value to change the air-fuel ratio to a leaner condition, in other words, at a value to decrease the fuel supplying amount. This allows the air-fuel ratio to be controlled to a desired value, for example, to a stoichiometric value. When the throttle valve is closed during engine warm-up, the intake manifold vacuum pressure is very high. As a result, the fuel vaporizes very well and the engine air-fuel ratio becomes richer. Under such conditions, the air-fuel ratio correction factor is maintained by the closed-loop at a value to greatly change the air-fuel ratio to a leaner condition.
If the engine is accelerated under the above conditions since the air-fuel ratio correction factor has been maintained at a value to change the engine air-fuel ratio to a leaner condition, the fuel is continues to be decreased inspite of the fuel-increment operation at acceleration (acceleration increment correction). As a result, the engine is not supplied with sufficient fuel for acceleration for a while. This causes sluggish acceleration and backfire. In other words, according to the above-mentioned conventional system, good acceleration characteristics cannot be obtained.